Christopher was with him because Eddie dropped him off as part of an effort to get Buck out of his post-quitting funk, which involved binge-watching food TV shows.
Buck decided to take Christopher for a day of ill-timed fun at the Santa Monica Pier.
Buck wasn’t at work with the LAFD due to the fact that Bobby wanted to stick him behind a desk after Buck nearly died from training-induced blood clots in his efforts to be restored to duty as a firefighter, leading Buck to just go ahead and quit the LAFD altogether.